Just a guess. Tight vacuum system and perhaps less chance of wear in the bearings. Pump may not have been designed with a constant duty cycle intended. Dead giveaway is a car that will not hold vacuum overnight perhaps.

Lack of oil changes will put more soot etc through them than manufacture intended as well. I always thought that keeping an old car up goes a long ways to keeping it reliable.

A road breakdown can prove far more expensive today than keeping the car up. Plus in my experience a road breakdown. Not always but often involves a serious loss of time factor. Especially on a trip. A service function was available at almost every gas station at one time. That is ancient history now.

Every area in the 616 and 617 cars that can cause reliability problems is very well known.

These bearings operate in a different way than usual. A constant back and forth movement of the balls. Has perhaps an unusual loading effect of the bearing cages. My best guess as I know next to nothing. Six ball bearings where used to allow stronger cages. So keeping the vacuum pump in good condition perhaps means keeping the vacuum system tight. The only item in the system I can think of designed to lose vacuum. There is probably a better term for that. Is the power brake canister.

Easy test I think is letting the car sit for a few days. Doors locks should still work. Most preventative maintainance on these old cars is more time intensive than money intensive.